Substances similar to insulin, ACTH and somatostatin are present in unicellular organisms and higher plants. The studies have been extended to further characterize the insulin-ACTH- and somatostatin-related molecules in spinach, E. coli and Saccharomyces. Using gel chromatography and high performance liquid chromatography the extracted materials have been purified in preparation for amino acid sequencing. To isolate the genes encoding these peptide hormones in multicellular non-vertebrates and unicellular organisms recombinant DNA technology is being used. Rat insulin-like growth factor I cDNA was cloned and sequenced to be used as an additional tool in the search for insulin-related genes in primitive eukaryotes and prokaryotes. Another use of this probe has been to study the regulation of IGF-I gene in rat. Differential splicing of primary transcript of rat IGF-I gene produces IGF-I mRNAs with at least three different 5'-untranslated region sequences. The relative steady state levels are differentially regulated in vivo by growth in tissue-specific manner.